Binding Enums to a ComboBox in Silverlight

The Enum class in Silverlight does not support the handy GetValues() method – so how to convince a ComboBox to present a list of all the enum values?

A first answer is quickly found: get the enum values via Enum.GetFields(), the code is rather simple:

object[] enumValues = typeof (ExperienceLevel).GetFields()
   .Where (f => f.IsLiteral)
   .Select (f => f.GetValue (null))
   .ToArray();

But how to bind it to the ComboBox? Assuming that you use the MVVM pattern, the first impulse would be to write something like this in XAML:

<ComboBox ItemsSource="{Binding MyEnumValues, Mode=OneTime}"
                SelectedItem="{MyEnumProperty, Mode=TwoWay}"/>

However, if we were to apply it we’d soon find two issues:

• The ComboBox will probably be blank, wrongly indicating that your property has not been set yet.
• The ViewModel class would now contain beside the actual data properties also the collection of enum values. That doesn’t feel quite right…

The first one comes from the fact that the enum values are converted into objects. Saving the converted “enum objects” in a static field will fix this issue quickly.

The other concern can actually be very neatly resolved by introducing a generic EnumValuesConverter. The idea is that you bind your XAML to the same property as your selected item, the converter detects then the property type and returns the enum values.

/// <summary>
/// Caches the "enum objects" for the lifetime of the application.
/// </summary>
internal static class EnumValueCache
{
	private static readonly IDictionary<Type, object&#91;&#93;> Cache = new Dictionary<Type, object&#91;&#93;>();

	public static object[] GetValues (Type type)
	{
		if (!type.IsEnum)
			throw new ArgumentException ("Type '" + type.Name + "' is not an enum");

		object[] values;
		if (!Cache.TryGetValue (type, out values))
		{
			values = type.GetFields()
				.Where (f => f.IsLiteral)
				.Select (f => f.GetValue (null))
				.ToArray();
			Cache[type] = values;
		}
		return values;
	}
}

/// <summary>
/// Enum => EnumValues
/// </summary>
public class EnumValuesConverter : IValueConverter
{
	public object Convert (object value, Type targetType, object parameter, CultureInfo culture)
	{
		if (value == null)
			return null;
		else
			return EnumValueCache.GetValues (value.GetType());
	}

	public object ConvertBack (object value, Type targetType, object parameter, CultureInfo culture)
	{
		throw new NotImplementedException();
	}
}

And your XAML code would be simplified to:

...
<local:EnumValuesConverter x:Name="enumConverter"/>
...
<ComboBox ItemsSource="{Binding MyEnumProperty, Mode=OneTime, Converter={StaticResource enumConverter}}"
                SelectedItem="{Binding MyEnumProperty, Mode=TwoWay}"/>

That’s it! Quite a neat solution and even reusable without any changes. Enjoy!